fix(iso): auto-detect kernel ABI at build time instead of pinning

DEBIAN_KERNEL_ABI=auto in VERSIONS — build.sh queries
apt-cache depends linux-image-amd64 to find the current ABI.
lb config now uses linux-image-amd64 meta-package.

This prevents build failures when Debian drops old kernel packages
from the repo (happens with every point release).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/auto/config

@@ -19,7 +19,7 @@ lb config noauto \
     --mirror-binary "https://deb.debian.org/debian" \
     --security true \
     --linux-flavours "amd64" \
-    --linux-packages "linux-image-${DEBIAN_KERNEL_ABI}" \
+    --linux-packages "linux-image-amd64" \
     --memtest none \
     --iso-volume "BEE" \
     --iso-application "Bee Hardware Audit" \